In numerous applications, it is desired to detect as soon as possible the growth of microorganisms such as bacteria or yeasts on a culture medium, usually a nutrient gel surface. In general, the surface of the gelose culture medium departs significantly from being accurately planar, since it frequently presents local depth defects of a few micrometers (μm) extending over distances of a few millimeters (mm). Dust or debris conveyed together with the sample may also give rise to local surface deformations that present high spatial frequencies.
Bacteria and yeasts are difficult to identify at the beginning of their growth, since they absorb very little light in the visible, the near ultraviolet, or the near infrared, and since their refractive index is very close to that of the surrounding medium. Thus, typically only colonies presenting a diameter greater than 100 μm can be detected by the naked eye; the time needed for colonies to grow to such dimensions is typically of the order of 6 hours (h) to 24 h.
Examination with a high power microscope, preferably against a black background, is one possible approach, but it is difficult to implement.
Other techniques are commonly used.
For example, it is possible to make the microorganisms fluorescent by means of various non-fluorescent additives that are transformed by the metabolism of the microorganisms into fluorescent substances, which substances are selected to remain inside the microorganisms for a long time. That method requires a considerable amount of time (greater than 5 h) before the bacteria become fluorescent, and it also requires fluorescent-generating metabolites to be developed.
Similarly, color-generating media enable microorganisms to be viewed in selective manner, but the same problem arises: staining requires a considerable length of time (several hours) before it is visible to the naked eye.
Furthermore, all of those methods run the risk of leading to serious disturbances of the metabolism of microorganisms, while subsequent tests (e.g. measuring sensitivity to antibiotics) require microorganisms to be developed under the most favorable possible conditions.